The present invention relates to an image forming apparatus which performs double-sided image formation on a sheet-shaped medium using the electrophotographic method, such as in a copier, a printer, a facsimile, etc., wherein toner images formed on an image bearing element are transferred and fixed onto both surfaces of the sheet-shaped medium after processing of the charging means, the exposing means and the developing means which are arranged around the periphery of the image bearing element.
Heretofore, in duplex copying, employed has been a method in which the first image formed on the image bearing element is transferred and fixed onto the sheet-shaped medium, and then this material is temporarily stored in a reverse paper feeding device for duplex copying, wherefrom it is conveyed out, synchronized with the formation of the second image on the image bearing element, so that the second image is transferred and fixed onto the other side of the sheet-shaped medium.
In the abovementioned duplex copying apparatus, since the transportation of the sheet-shaped medium is carried out twice in such a manner as to feed it to the reverse paper feeding device and to again pass it through the fixing apparatus, the reliability of the transportation is decreased, resulting in unacceptably frequent jam occurrences. To cope with this problem, there has been proposed an apparatus which fixes toner images transferred on both surfaces of the sheet-shaped medium all at once, after transferring operation, set forth in Tokkoushou 49-37535, 54-28740, Tokkaihei 1-44457, 4-21576, etc. Especially, Tokkaihei 1-44457, 4-21576, etc. have disclosed a method for forming a duplex copy with color images by means of a plurality of image forming means comprising a image bearing element, a charging means, an image exposing means and a developing means, which are arranged in parallel to a toner image receiving member.
The applicants of the present invention has been studying a duplex image forming method wherein a toner image formed on an image bearing element (a first image bearing means) is temporarily transferred onto an intermediate transfer element (a second image bearing means) one at a time, and, after another toner image is formed again on the first image bearing element, both toner images are transferred simultaneously onto both surfaces of the sheet-shaped medium. There have been problems, however, in the difficulty of the sheet-shaped medium separating from the image bearing elements which caused transportation defects and/or deformation of the sheet-shaped medium, causing a scattering of toner on the unfixed transferred image.